Tag generation method in broadcast encryption system

ABSTRACT

A tag generation method for generating tags used in data packets in a broadcast encryption system is provided. The method includes detecting at least one revoked leaf node; setting a node identification (node ID) assigned to at least one node among nodes assigned node IDs at a layer 0 and to which the at least one revoked leaf node is subordinate, to a node path identification (NPID) of the at least one revoked leaf node at the layer 0; generating a tag list in the layer 0 by combining the NPID of each of the at least one revoked leaf nodes at the layer 0 in order of increment of node IDs of the corresponding at least one revoked leaf nodes; and generating a tag list in a lowest layer by repeatedly performing the setting and generation operation down to the lowest layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.13/278,140, filed in the U.S. Patent and Trademark Office on Oct. 20,2011, which is a continuation of U.S. application Ser. No. 11/406,254,filed in the U.S. Patent and Trademark Office on Apr. 19, 2006, now U.S.Pat. No. 8,055,896, issued on Nov. 8, 2011, which claims the benefitunder 35 U.S.C. §119 (a) from U.S. Provisional Patent Application No.60/672,550, filed in the U.S. Patent and Trademark Office on Apr. 19,2005, and priority from Korean Patent Application No. 10-2005-0117724,filed on Dec. 5, 2005, in the Korean Intellectual Property Office, theentire disclosures of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Methods and apparatuses consistent with the present invention relate totag generation in a broadcast encryption (BE) system. More particularly,the present invention relates to a tag generation method in a BE systemfor efficiently reducing a tag size.

2. Description of the Related Art

The broadcast encryption (BE) system enables a transmitter, that is, abroadcast center, to effectively transmit information only to intendedusers among all users. The BE should be available effectively whenever aset of the intended users arbitrarily and dynamically changes. Animportant property of the BE is to revoke or exclude an unintendeddevice or user, for example, an illegal user or an expired user.

In order to revoke or exclude an unintended device or user, each devicestores a different key set assigned to that particular device, and aservice provider stores the whole key set of the all devices.

Various schemes have been suggested for such a BE system. Generally, theBE system employs a layered node structure. Alternatively, the BE systemmay be implemented using a hierarchical hash-chain broadcast encryptionscheme (HBES).

FIG. 1 depicts how to assign keys to nodes, respectively, in aconventional BE system. Referring to FIG. 1, nodes 0 through 3 arearranged in a circle. The respective nodes 0 through 3 correspond tousers in the BE system. Each node i is assigned a unique node key Ki. Inother words, the node key K0 is assigned to the node 0, the node key K1is assigned to the node 1, the node key K2 is assigned to the node 2,and the node key K3 is assigned to the node 3.

To enable private communications between or among authorized users, acertain key shared only by the authorized users should be assigned tothe nodes of the circular structure. For doing this, the unique keysassigned to the nodes are consecutively applied to a one-way hashfunction to generate key values, that is, key sets. The generated keyvalues are assigned to the nodes, respectively, in a manner as shown inTable 1.

TABLE 1 Node 0 Node 1 Node 2 Node 3 Key set K0 H (K0) HH (K0) HHH (K0)HHH (K1) K1 H (K1) HH (K1) HH (K2) HHH (K2) K2 H (K2) H (K3) HH (K3) HHH(K3) K3

In Table 1, ‘H’ denotes the one-way hash function, and HH(K0)=H(H(K0)).The one-way hash function takes an input value of an arbitrary lengthand produces an output value of a fixed length. The one-way hashfunction has properties such that it is infeasible to find the inputvalue using a given output value, and it is impossible to find anotherinput value that produces the same output value as a given input value.In addition, it is impossible to find two different arbitrary inputvalues that produce the same output value.

As mentioned above, the hash function is a function that isadvantageously applied for data integrity, authentication, repudiationprevention, and the like. The one-way hash function may be HBES SHA −1.

Referring back to FIG. 1, in case that only the nodes 0, 1 and 2 want tosecure a safe, or private, communication channel, they use HH(K0) as anencryption key. In doing so, the nodes 0, 1 and 2 may store HH(K0)corresponding to the encryption key or easily compute HH(K0) using astored value. However, the node 3 cannot compute HH(K0) corresponding tothe encryption key, using its stored HHH(K0).

Thus, a node excluded from the encryption communication channel, such asthe node 3 in the above example, is referred to as a revoked node, and anodes constructing the private communication channel are referred to asa privileged nodes. Therefore, in the above example, nodes 0, 1 and 2would be the privileged nodes. The set of the nodes arranged in a circleare referred to as a node group.

To handle a large number of nodes, it is necessary to layer thestructure of FIG. 1.

FIG. 2 depicts a layered structure of the circular node groups of FIG.1.

As shown in FIG. 2, two layers of a layer 0 and a layer 1 are shown, anda node group at each layer consists of 4 nodes. The respective nodes areassigned the key values or key sets generated using the hash function ina manner as shown in Table 1. The nodes at the lowest layer 1 are leafnodes.

Note that the nodes at the lower layer hold keys assigned to theirparent nodes in the upper layer in the layered structure of FIG. 2. Inaddition, when a node is revoked from the communication channel, theparent node of the revoked node is also regarded as the revoked node.

For example, the node 3 of the node group 1 stores its assigned key setand the key set of the node 0 in the node group 0. If the node 1 of thenode group 3 is revoked, the node 2 of the node group 0 is also regardedas the revoked node.

In the example, the nodes 3, 0 and 1 of the node group 0 can secure theencryption communication channel by using HH(K03), which is generatedfrom the encryption key of the node 3 of the node group 0 K03 (0 denotesthe number of the node group and 3 denotes the serial number of thenode), as the encryption key.

The privileged nodes in the node group 3 can also secure the encryptioncommunication channel by using HH(K32) generated from K32 as theencryption key.

Accordingly, a server is able to transmit the encrypted information toall the nodes but the node 1 of the node group 3 using HH(K03) andHH(K32) as the encryption key.

That is, the server transmits to the leaf nodes a temporary keyencrypted using the selected encryption key as aforementioned, andcontent encrypted with the temporary key.

Upon receiving the encrypted data packets from the server, the leafnodes require information as to which one of its stored keys is used togenerate the encryption key and to decrypt the data packet.

Hence, when transmitting the encryption key, the server appends a tag tothe data packets so that the leaf nodes can acquire the informationrelating to the encryption key. The tag contains information relating tothe revoked nodes.

Thus, the leaf nodes can learn the encryption key of the received datapackets and thus generate the encryption key by means of the informationrelating to the revoked nodes.

As the above examples illustrate, a transmission overhead, a storageoverhead, and a computation overhead are necessary in the BE. Thetransmission overhead is a quantity of the header transmitted from thetransmitter, the storage overhead is a quantity of a secret key storedby the user, and the computation overhead is a quantity of computationrequired for the user to acquire a session key. It is thereforedesirable to reduce the overhead in the BE system, and specifically toreduce the transmission overhead according to the tag transmission.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided a taggeneration method in a BE system which takes advantage of efficientgeneration of a node ID of a revoked leaf node to reduce a tag size.

In accordance with an aspect of the present invention, a tag generationmethod for generating tags used in a broadcast encryption system, whichhas a layered structure and which includes a plurality of node groupseach consisting of a plurality of nodes, comprises detecting at leastone revoked leaf node; setting a node identification (ID) assigned to atleast one node among nodes assigned node IDs at a layer 0 and to whichthe at least one revoked leaf node is subordinate, to a node pathidentification (NPID) of the at least one revoked leaf node at the layer0; generating a tag list in the layer 0 by combining the NPID of each ofthe at least one revoked leaf nodes at the layer 0 in order of incrementof node IDs of the corresponding at least one revoked leaf nodes; andgenerating a tag list in a lowest layer by repeatedly performing thesetting and generation operation down to the lowest layer. The NPID maybe combined with a group identifier (GID) indicative of information asto a parent node of a node corresponding to the NPID.

A first NPID at each layer may be combined with a GID 0.

In the same node group as a previous NPID, a NPID from a second NPID ateach layer may be combined with the same GID as is combined with theprevious NPID.

In a different node group from a previous NPID, a NPID from a secondNPID at each layer may be combined with a GID which is a remainder afteradding 1 to the previous NPID and dividing by 2.

The NPID may be combined with a GID of a parent node of a nodecorresponding to the NPID.

The node ID may be assigned as a hexadecimal, and the node group mayinclude 16 nodes.

The lowest layer may be a layer 15.

When all leaf nodes along lower branches from a certain node in a treetopology are revoked, an NPID of the revoked leaf nodes may besubstituted by a smallest NPID of the NPIDs.

The smallest NPID used for the substitution may be combined with abinary GID where ‘1s’ as many as a certain number are consecutivelyarranged.

The certain number may be a log to a base 2 of a number of nodes in anode group including a node corresponding to the NPID.

A combination of NPIDs at each layer with respect to the at least onerevoked leaf node may be a node ID of the at least one revoked leafnode.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

These and other aspects of the present invention will become moreapparent from the following description of exemplary embodimentsthereof, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram illustrating assigning keys to nodes in aconventional BE system.

FIG. 2 is a diagram illustrating a layered structure of circular nodegroups of FIG. 1;

FIG. 3 is a diagram illustrating a layered structure adopting a taggeneration method according to an exemplary embodiment of the presentinvention;

FIG. 4 is a diagram illustrating the revocation of all leaf nodessubordinate to a node at a certain layer according to an exemplaryembodiment of the present invention; and

FIG. 5 is a graph illustrating the tag size according to the taggeneration method according to an exemplary embodiment of the presentinvention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE PRESENT INVENTION

Certain exemplary embodiments of the present invention will now bedescribed in greater detail with reference to the accompanying drawings.

In the following description, the same drawing reference numerals areused to refer to the same elements, even in different drawings. Thematters defined in the following description, such as detailedconstruction and element descriptions, are provided as examples toassist in a comprehensive understanding of the invention. Also,well-known functions or constructions are not described in detail, sincethey would obscure the invention in unnecessary detail.

FIG. 3 depicts a layered structure adopting a tag generation methodaccording to an exemplary embodiment of the present invention.

FIG. 3 shows that the layered structure consists of three layers 0, 1and 2. However, any number of layers may be used. Note that each nodegroup in FIG. 3 may be a circular node group as shown in FIG. 2. To easethe understanding, the circular formation is not illustrated in thedrawings.

Referring now to FIG. 3, the layer 0 includes a node group consisting of16 nodes. The layer 1 has a child node group built up with node groupseach consisting of 16 nodes, each node group of layer 1 corresponding toa respective one of the 16 nodes at the layer 0. The node groups oflayer 1 are subordinate to the 16 nodes of the layer 0, respectively. Inother words, there are 16 node groups each consisting of the 16 nodes atthe layer 1, and accordingly, 16² nodes are present in total.

At the layer 2, a child node group includes node groups each consistingof 16 nodes for each of the respective 16² nodes at the layer 1. Inother words, since there are 16² node groups each consisting of the 16nodes are present at the layer 2, 16³ nodes are present in total.Herein, the 16³ nodes at the lowest layer 2 are referred to as leafnodes.

In this exemplary embodiment of the present invention, the layeredstructure may include 16 layers, that is, layers 0 through 15. In thiscase, the layer 15 has a child node group built up with node groups eachconsisting of 16 nodes for the respective 16¹⁵ nodes at the layer 14.That is, there are 16¹⁵ node groups each consisting of 16 nodes at thelayer 15, and accordingly, 16¹⁶ nodes, that is, 16¹⁶ leaf nodes arepresent in total.

Hereafter, how to determine a node identifier (node ID) of a leaf nodeis described in detail.

In this exemplary embodiment of the present invention, hexadecimals from0 to F are assigned to the nodes in each node group of FIG. 3 accordingto an order, as their serial numbers. Provided that the number of nodesin each node group is N, the serial numbers from 0 to N−1 are assignedto the nodes in each node group.

In FIG. 3, according to the tag generation method of an exemplaryembodiment of the present invention, the node ID of the leaf node is aconsecutive arrangement of the serial numbers assigned to the nodes, towhich the leaf node is subordinate, at the layers 0 through 15.Hereafter, the serial number at each layer is referred to as a node pathidentifier (NPID) at each layer with respect to the corresponding leafnode. In conclusion, the arrangement of the NPIDs at the layers is thenode ID of the corresponding leaf node.

Table 2 shows the determination of the node ID of the leaf nodeaccording to the tag generation method of the present invention.

TABLE 2 i ii iii iv v vi vii viii ix Layer 0 1 1 1 1 1 8 8 8 8 Layer 1 BB B B B 0 0 0 F Layer 2 1 2 B C D 2 6 B 8

Still referring to FIG. 3, the leaf node indicated by solid circle atthe layer 2 denotes the revoked leaf node. Provided that 9 nodes arerevoked in total, the node ID of each revoked leaf node is createdaccording to the following scheme.

-   -   Priority 1: the order from the upper layer to the lower layer.    -   Priority 2: at the same layer, the smaller NPID assigned to the        parent node.    -   Priority 3: in the same node group, the smaller NPID of the        corresponding node group.

According to the priorities, in case of the node ID of the revoked leafnode i, a NPID ‘1’ is assigned to its parent node at the layer 0 beingthe highest layer. ‘1’ becomes the NPID of the revoked leaf node i atthe layer 0. Next, the NPID ‘B’ is assigned to the parent node of thenode i, at the layer 1. ‘B’ becomes the NPID of the revoked leaf node iat the layer 1. Lastly, the NPID ‘1’ is assigned to the revoked leafnode i at the layer 2. ‘1’ becomes the NPID of the revoked leaf node iat the layer 2. As such, the node ID of the revoked leaf node isdetermined to be [1, B, 1].

As for the node ID of the revoked leaf node vi, the parent node of thenode vi, at the layer 0 being the highest layer, is assigned a NPID ‘8’.‘8’ becomes the NPID of the revoked leaf node vi at the layer 0. Next,the parent node of the node vi, at the layer 1, is assigned a NPID ‘0’.‘0’ becomes the NPID of the revoked leaf node vi at the layer 1. Lastly,an NPID ‘2’ is assigned to the revoked leaf node vi at the layer 2. ‘2’becomes the NPID of the revoked leaf node vi at the layer 2. As such,the node ID of the revoked leaf node vi is determined to [8, 0, 2].

Table 3 shows the rearrangement in the line writing direction of thedetermined node IDs of the revoked leaf nodes of Table 2.

TABLE 3 Layer 0 Layer 1 Layer 2 1 1 1 1 1 8 8 8 8 B B B B B 0 0 0 F 1 2B C D 2 6 B 8

However, in practice, when transmitting the tag information to the leafnode, a group ID (GID) is appended to the NPID. Table 4 shows thecombination of the GID according to an exemplary embodiment of thepresent invention.

TABLE 4 Layer 0 Layer 1 Layer 2 GID 0 0 0 0 0 0 0 0 0 1 1 1 1 1 8 8 8 8B B B B B 0 0 0 F NPID 1 1 1 1 1 8 8 8 8 B B B B B 0 0 0 F 1 2 B C D 2 6B 8

In Table 4, the GID combined with the NPID at each layer becomes theNPID of the parent node of the node at each layer corresponding to therevoked leaf node. Note that the GID at the layer 0 is ‘0’ because thenode corresponding to the revoked leaf node, at the layer 0, has noparent node.

That is, the NPID is combined with the GID that is the NPID of theparent node of the node corresponding to the NPID.

Table 5 shows tag tables transmitted to the leaf nodes when the GIDs arecombined as shown in Table 4.

TABLE 5 Tag 01 01 01 01 01 08 08 08 08 Layer 0 table 1B 1B 1B 1B 1B 8080 80 8F Layer 1 B1 B2 BB BC BD 02 06 0B F8 Layer 2

Table 6 shows the combination of the GID according to an alternativeembodiment of the present invention.

TABLE 6 Layer 0 Layer 1 Layer 2 GID 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 1 10 0 0 0 0 1 1 1 0 NPID 1 1 1 1 1 8 8 8 8 B B B B B 0 0 0 F 1 2 B C D 2 6B 8

In Table 6, the first NPID at each layer is combined with the GID ‘0’.The NPID from the second NPID at each layer is combined with the sameGID as the GID of the previous NPID within the same node group as theprevious NPID.

By contrast, in the different node group from the previous NPID, theNPID after the second NPID at each layer is combined with the GID thatis the remainder after adding ‘1’ to the previous NPID and dividing itby ‘2’. More specifically, in case that the NPID from the second NPID ateach layer is in the different node group from the previous NPID, theprevious GID ‘1’ becomes ‘0’ and the previous GID ‘0’ becomes ‘1’.

Table 7 shows tag tables transmitted to the respective leaf nodesaccording to the GID combination method as shown in Table 6.

TABLE 7 Tag 01 01 01 01 01 08 08 08 08 Layer 0 table 0B 0B 0B 0B 0B 1010 10 1F Layer 1 01 02 0B 0C 0D 12 16 1B 08 Layer 2

In this exemplary embodiment of the present invention, it can be assumedthat all leaf nodes subordinate to a node at the specific layer arefully revoked.

FIG. 4 depicts the full revocation of all leaf nodes subordinate to anode at the specific layer according to an exemplary embodiment of thepresent invention.

In FIG. 4, the layered structure consists of three layers 0, 1 and 2.However, this is only an example, and the layered structure may have anynumber of layers. As shown in FIG. 4, the layer 0 includes a node groupconsisting of 4 nodes. The layer 1 has a child node group built up withnode groups each consisting of 4 nodes for the 4 nodes at the layer 0,respectively. In other words, since there are 4 node groups eachconsisting of the 4 nodes at the layer 1, 4² nodes are present in total.

At the layer 2, a child node group is built up with node groups eachconsisting of 4 nodes for the 4² nodes at the layer 1, respectively. Inother words, since there are 4² node groups each consisting of the 4nodes at the layer 2, 4³ nodes are present in total. Herein, the 4³nodes at the lowest layer 2 are referred to as leaf nodes.

In this exemplary embodiment of the present invention, the layeredstructure may include 16 layers of layers 0 through 15. Accordingly,there would be 16 nodes in each node group. In this case, the layer 15has a child node group built up with node groups each consisting of 16nodes for the respective 16¹⁵ nodes at the layer 14. That is, there are16¹⁵ node groups each consisting of 16 nodes at the layer 15, andaccordingly, 16¹⁶ nodes, that is, 16¹⁶ leaf nodes are present in total.

Still referring to FIG. 4, as one can see, all leaf nodes subordinate tothe second node in the node group at the layer 0 are revoked, and one ofthe leaf nodes subordinate to the fourth node in the node group at thelayer 0 is revoked.

Table 8 shows the node ID of the revoked leaf nodes in accordance withTable 2.

TABLE 8 Leaf node a b c d e f g h i j k l m n o p q Layer 0 1 1 1 1 1 11 1 1 1 1 1 1 1 1 1 3 Layer 1 0 0 0 0 1 1 1 1 2 2 2 2 3 3 3 3 1 Layer 20 1 2 3 0 1 2 3 0 1 2 3 0 1 2 3 3

In reference to FIG. 4 and Table 8, the parent nodes of the revokednodes are in the same group at the layer 1. These parent nodes at thelayer 1 have the common parent node at the layer 0.

In this exemplary embodiment of the present invention, the node IDs ofthe revoked leaf nodes a through p are substituted by the node ID of therevoked leaf node a. The substituted node IDs are shown in Table 9.

TABLE 9 Leaf node a~p q Layer 0 1 3 Layer 1 0 1 Layer 2 0 3

In event that all leaf nodes, which are subordinate to lower branchesfrom a specific node, are revoked in the layered structure, the NPID ofthe revoked leaf nodes is substituted by the smallest NPID among theNPIDs of the revoked leaf nodes at the respective layers.

Table 10 show the GID combination method in FIG. 4.

TABLE 10 Layer 0 Layer 1 Layer 2 GID 0 0 1111(2) 0 1111(2) 0 NPID 1 3 01 0 3

Referring to Tables 8, 9 and 10, the node ID [1, 0, 0] which substitutesthe node IDs of the revoked leaf nodes a through p, (hereafter, referredto as a representative node ID) consists of the NPIDs [1], [0] and [0].Among them, while the NPID [1] at the layer 0 was the duplicate NPID ofthe revoked leaf nodes a through p, the NPIDs [0] and [0] at the layers1 and 2 substitute for the NPIDs [0], [1], [2] and [3] of the leaf nodesa through p at the layers 1 and 2, respectively.

Of the NPIDs constituting the representative node ID, the NPID [1] atthe layer 0 has no substituting NPID. Thus, the substitution is notindicated. Instead, in the manner as shown in Table 6, the NPID [1] iscombined with the GID ‘0’ as the first NPID at the layer.

Of the NPIDs constituting the representative node ID, the NPIDs [0] and[0] at the layers 1 and 2, respectively, substitute for the NPIDs [0],[1], [2] and [3] of the leaf nodes a through p. To represent thissubstitution, a binary GID in which ‘1’s as many as a certain number areconsecutively arranged, for example, 11, . . . , 11₍₂₎, is combined.

In this embodiment of the present invention, the cipher of the GID maybe determined according to the number of types of the substituted NPIDs.When four types of the NPIDs are substituted as shown in FIG. 4, the GIDis 11₍₂₎. On the other hand, provided that the node group consists of 16nodes, the GID is 1111₍₂₎.

It can be said that the cipher of the GID is log₂ t wherein t is thenumber of nodes in the node group to which the node corresponding to theNPID of the representative node ID belongs.

In Table 10, aside from the NPIDs constituting the representative nodeID, the GID of other NPID is determined as shown in Table 6.

Specifically, within the same node group as the previous NPID, the NPIDat the layer is combined with the same GID as combined to the previousNPID.

By contrast, in the different node group from the previous NPID, theNPID from the NPID at the layer is combined with the GID that is theremainder after adding ‘1’ to the previous NPID and dividing it by ‘2’.

Table 11 shows tag tables transmitted to the leaf nodes when the GID iscombined in the manner as shown in Table 10.

TABLE 11 Tag table 01 03 Layer 0 11 (2) 0 01 Layer 1 11 (2) 0 03 Layer 2

FIG. 5 is a graph comparing the tag size between the tag generationmethod according to an exemplary embodiment of the present invention andthe conventional tag generation method as disclosed in U.S. PublishedPatent Application No. 20020147906.

As shown in FIG. 5, it is apparent that the tag size 100 of an exemplaryembodiment of present invention is much smaller than the tag size 200 ofthe above literature. In case that only one leaf node is revoked, themethod according to an exemplary embodiment of present invention canreduce the tag size by about 65 times than the above literature. In caseof 16 revoked leaf nodes, the method according to an exemplaryembodiment of the present invention can reduce the tag size by about 61times that of the above literature.

In addition, as for 256 revoked leaf nodes, the tag size is reduced byabout 57 times, and as for 65,536 revoked leaf nodes, the tag size isreduced by about 49 times. As for 4.2 billion revoked leaf nodes, thetag size is reduced by about 32 times.

As set forth above, exemplary embodiments of the present invention candrastically reduce the transmission overhead at the server in the BEsystem owing to the reduced tag size.

Although a few exemplary embodiments of the present invention have beenshown and described, it would be appreciated by those skilled in the artthat changes may be made in these exemplary embodiments withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents.

1. A tag generation method used in an encryption system which has a layered structure and includes a plurality of node groups each consisting of a predetermined number of nodes, the method comprising: detecting at least one excluded leaf node; recognizing a node identification (ID) assigned to parent nodes of the detected left node; and generating a tag list in which the recognized node IDs of parent nodes and node ID generated by arranging a node ID of the excluded leaf node are recorded as an excluded node ID, wherein the plurality of node groups are circular node groups.
 2. The tag generation method of claim 1, wherein the nodes are layered from 0 layer to n−1 layers, wherein the leaf node belongs to the n−1 layer, wherein IDs of the parent nodes comprise ‘an ID of a parent node which belongs to 0 layer’, ‘an ID of a parent node which belongs to 1 layer’, . . . and ‘an ID of a parent node which belongs to n−2 layer’.
 3. The tag generation method of claim 1, wherein the nodes are grouped, and wherein the ID comprises a node path ID which is determined according to an order of the node in a group containing a node.
 4. The tag generation method of claim 1, wherein the generating comprises generating a tag list of the excluded nodes by combining 0) IDs of parent nodes at 0 layer regarding excluded leaf nodes, 1) IDs of parent nodes at 1 layer regarding the excluded leaf nodes, 2) IDs of parent nodes at 2 layer regarding the excluded leaf nodes, . . . , n−2) IDs of parent nodes at n−2 layer regarding the excluded leaf nodes, and n−1) IDs of the excluded leaf nodes which belong to n−1 layer.
 5. The tag generation method of claim 1, wherein the node ID is assigned as a hexadecimal, and wherein the node group comprises 16 nodes, respectively.
 6. A tag generation method used in an encryption system which has a layered structure and includes a plurality of node groups each consisting of a predetermined number of nodes, the method comprising: detecting at least one of excluded first leaf node and second leaf node; recognizing a node ID of first parent nodes of the detected first leaf node; recognizing a node ID of second parent nodes of the detected second leaf node; and generating a tag list where node IDs generated by arranging the recognized node ID of first parent nodes and a node ID of the first leaf node as an excluded first leaf node ID, wherein the plurality of node groups are circular node groups.
 7. The tag generation method of claim 6, wherein the nodes are layered from 0 layer to n−1 layers, wherein the first leaf node and the second leaf node belong to the n−1 layer, and wherein the generating comprises generating a list of the excluded nodes by arranging ‘an ID of a first parent node which belongs to 0 layer’, ‘an ID of a second parent node which belongs to 0 layer’, ‘an ID of a second parent node which belongs to 1 layer’, . . . , ‘an ID of a first parent node which belongs to n−2 layer’, ‘an ID of a second parent node which belongs to n−2 layer’, and an ID of the first leaf node, and an ID of the second leaf node.
 8. The tag generation method of claim 7, wherein the nodes are grouped, and wherein the ID is generated as ‘a node path ID which is determined according to an order of the node in a group containing a node’ and ‘a group ID which indicates a group containing the node’ are combined.
 9. The tag generation method of claim 6, wherein the node ID is assigned as a hexadecimal, and wherein the node group comprises 16 nodes, respectively. 